The present invention relates generally to overlays of organic polymer films, the volumes of which are provided with "lossy" components for dispersal on metal foil to render the foil "active" for microwave heating and cooking. The invention relates, in addition, to a methodology for predicting the heating behavior of the combination of film and foil, and a thin impedance matching film.
Metal foil and metal containers have certain attractive features in regard to the heating of certain "loads", particularly the heating and cooking of food loads, which are dielectric in nature. For example, an aluminum foil or container has excellent "barrier" properties, which is generally not the case with microwave transparent and semi-transparent packages and containers, made from paper, plastics and metalized paper materials. Paper and plastics are generally penetrateable by ambient light and are somewhat porous to the atmosphere outside the container. Metal containers, of course, form a complete barrier to light and the atmosphere external to the container.
In the case of semi-transparent films, which currently use low bulk or surface loadings of active dielectric materials (including metals, semi-metals, organic or inorganic semi-conductors, and electrically insulating organic or inorganic compounds) for heating, the polymer matrix of the film, which is a poor heat conductor, dominates thermal transport properties. Low thermal transport causes the average film temperature to run high, even to self destruction, unless the film is everywhere well heat-sinked, or the structure of the absorbing particles or surface islands coincidently alters upon reaching a safe maximum design temperature to reduce energy absorption in the structure of the film.
Semi-transparent films loaded throughout their bulk with such dielectric particles, or surface loaded with such particles, as in the James River U.S. Pat. Nos. 4,626,641 and 4,590,349 to Brown, Canadian 1,53,069 and DuPont's U.S. Pat. No. 4,518,651 to Wolfe, distribute the power incident on the film between power reflected from the film, power transmitted through the film and power absorbed by the film in accordance with the intrinsic dielectric response of the materials and the film thickness. The physics governing energy partition amongst reflection, transmission and absorption (Maxwell's Equations) and the dielectric properties of available materials invariably forces a compromise between power absorption and film thickness in a given application. The particles in these cases are thin surface islands of metals or semi-metals, or these elements appear in some chemical arrangement with another element, such as oxygen or nitrogen.
For example, calculations show that polymer films loaded with an appropriate carbon in the range of 6 to 9% of wt/wt carbon/polymer may be made to absorb as much as 28% of the incident microwave power in a free standing mode, i.e., with no electrically conductive backing, with air being the medium on both sides of the film. (The effect of water or a food load on one side of a film is discussed below.) However, such values necessarily correspond to film thicknesses greater than 20 mils. Such high loadings are detrimental to both the mechanical properties of the films and their economics for packaging applications, and at such thicknesses, the inherent thermal transport properties of the films become a limiting factor to the viability of the films for cooking purposes.
For these technical reasons and for economic reasons, semi-transparent films are typically applied at five to ten mil thicknesses, with power absorption being on the order of three to eight percent (again, with ambient air existing on both sides of the film). The lower heat generation rates and inferior thermal transport rates combine to limit use of semi-transparent films to browning/crisping applications in food preparations, where the combination of elevated temperature and low heat transport are adequate to the task.
As a practical matter, transparent and semi-transparent laminated materials, as well as laminates using solid barrier metals, all are implemented under 37 boundary conditions" that affect the generation of thermal energy and its transport through the absorber structure to the load. For example, the electrical boundary conditions (or equivalently, the optical boundary conditions) imposed on the microwave absorbing film determine the governing equations describing the apportionment of incident microwave energy between reflection, absorption and transmission in the film; such quantities generally are referred to herein as the "optical response" of the absorbing structure. Calculating optical responses of the "D-layer" of the subject invention is discussed hereinafter on pages 18 and 19 of the present text. Similarly, the structures and temperatures of the film and the environment surrounding the film define thermal boundary conditions important to describing the transport of heat from the film where it is generated to the product or load.
The cited analysis provides a perspective on the heating performance of semi-transparent films, using air as a medium on both sides of the film. The analysis enables one to quote maximum anticipated film performance characteristics. The analysis further shows that the optical constants of a load adjacent to one side of a semi-transparent film will strongly influence microwave absorption in the film, since the film plus load will determine relative power absorption in the film versus power reflection at the surface of the film and power transmission through the film into the load.
For example, if water replaces air on the load side of a 5% carbon film five mils thick of the kind previously considered, the calculated power absorption in the film drops from 3% to less than 1%. This effectively eliminates the utility of the absorbing film. Each food load introduces its own constraint of this type for semi-transparent films. The influence of a load on the division of incident power between reflection, absorption, and transmission can be mitigated by the use of microwave transparent spacers, e.g., paperboard located between the semi-transparent film and load, as is done in a number of package designs, but only at the expense of aggravating the thermal transport/film temperature problem. In contrast thereto, there is no such consideration in a microwaveable metal foil strategy because the foil represents an electrical short circuit (or equivalently, a perfect reflector of a microwave) in its contribution to the net impedance of the absorber, thus eliminating any influence on the absorber performance from the optical characteristics of the load.
It is important to note that semi-transparent films having the bulk of the polymer or its surface loaded with dielectric particles or molecules will produce only second order effects on heating rates when these films are applied to an electrically conductive substrate like aluminum foil. ("Second order effects" in the present context means that the heat generation rate of the film is minor in magnitude and will not vary with differing film thicknesses over the sensible thickness range [0.5 to 20 mils] applicable in the packaging industry). This limitation is not encountered in the case of magnetic loadings. The limitation on dielectric films derives from the basic physics governing the interaction of an electromagnetic wave with dielectric material structures backed by a conductive substrate, and has been amply verified by experiments. The second order contribution that is observed with semi-transparent, dielectric films on aluminum foil most likely originates with the minor component of plane-polarized radiation, such polarization being defined by the electric component of a propagating electromagnetic wave that lies parallel to the plane of incidence to the foil surface in a multi-mode consumer oven. A possibly minor additional heating contribution may come from microwave scattering at the lossy particles which acts to enhance the optical path length within the absorbing layer while also altering the average polarization of the electromagnetic wave propagating through the lossy layer of the film/metal laminate.
Metal foil and metal containers have superior thermal conductivity, resulting in faster heatup times, than electromagnetic transparent materials, and can simultaneously minimize cooking hotspots that tend to occur with transparent and semi-transparent materials. If the metal is heated by a microwave absorbable layer located on the outside surface of the metal, the metal of the container is heated in a manner that emulates conventional thermal cooking. This provides good taste, odor, color and consistency of the food load cooked in such a container, and is expected to reduce or eliminate entirely the need to reconstitute foods specifically for microwave cooking or preparation.